In the field of oil production it is common practice to drill a number of branched, lateral wells or drains from a single master well. Such a technique is used for both technical and economic reasons.
Lateral wells drilled from a single, master well enable the oil-containing strata to be reached at a number of points, so increasing both the rate and the volume of production. Furthermore, the use of such a technique offers economic advantages as the lateral wells all share a common well-head and production outlet.
Another use of such drains is to make it possible to work strata having a large number of fractures. The drains each intercept a number of fractures and connect them to the drainage system of the master well.
Lateral drains are typically drilled from the master well at a point some distance below the surface by using a device known as a whipstock. This is a tool which serves to deviate the drillstring from the axis of the master well on to a new predetermined course. Having drilled a first lateral drain, the drillstring is removed and the whipstock is turned to a new orientation and depth within the well so as to deviate the drillstring, once it is returned within the well, on to a new course at a predetermined path relative to both the master well and the first drain.
The document U.S. Pat. No. 4,573,541 describes a device of this type. In particular, this document describes a drilling and petroleum production start-up device consisting of a master well and at least one branched well opening into the master well, and comprising an outer tube located in the master well, and at least one take-off assembly fastened in situ in the outer tube and at least one fixed take-off tube, the lower end of which communicates with a branched well. Typically, this device is adapted for drilling three branched wells from the master well.
While this device enables several branched wells to be simply drilled from a master well, it does present certain disadvantages. Specifically, as the three take-off tubes are arranged side by side, their diameters are necessarily small in relation to that of the master well. Similarly, should the diameter of the take-off tubes be required to be large, there is a corresponding increase in the dimensions of the whole device.
The document EP-A-0 574 326 describes a device enabling a lateral well to be drilled from a master well which comprises a curved surface arranged within the device and intended to deviate a drillstring onto a predetermined path, the drillstring passing through an opening formed in the wall of the device. However, this device does not resolve the problem of ensuring a complete seal between the outer wall of the lateral well and that of the master well.